Abstract

Plate impact experiments have been carried out to examine the influence of grain boundarycharacteristics on the dynamic tensile response of Cu samples with grain sizes of 30, 60, 100, and 200 μm. The peak compressive stress is ∼1.50 GPa for all experiments, low enough to cause an early stage of incipient spall damage that is correlated to the surrounding microstructure in metallographic analysis. The experimental configuration used in this work permits real-time measurements of the sample free surfacevelocity histories, soft-recovery, and postimpact examination of the damaged microstructure. The resulting tensile damage in the recovered samples is examined using optical and electron microscopy along with micro x-raytomography. The free surfacevelocity measurements are used to calculate spall strength values and show no significant effect of the grain size. However, differences are observed in the free surfacevelocity behavior after the pull-back minima, when reacceleration occurs. The magnitude of the spall peak and its acceleration rate are dependent upon the grain size. The quantitative, postimpact, metallographic analyses of recovered samples show that for the materials with grain sizes larger than 30 μm, the void volume fraction and the average void size increase with increasing grain size. In the 30 and 200 μm samples, void coalescence is observed to dominate the void growth behavior, whereas in 60 and 100 μm samples, void growth is dominated by the growth of isolated voids. Electron backscatter diffraction (EBSD) observations show that voids preferentially nucleate and grow at grain boundaries with high angle misorientation. However, special boundaries corresponding to Σl (low angle, < 5 °) and Σ3 (∼60 ° <111> misorientation) types are more resistant to void formation. Finally, micro x-raytomography results show three dimensional (3D) views of the damage fields consistent with the two dimensional (2D) surface observations. Based on these findings, mechanisms for the void growth and coalescence are proposed.

Received 01 February 2011Accepted 02 June 2011Published online 05 August 2011

Acknowledgments:

This work was supported by LDRD-DR 20100026. The authors would like to thank Mike Lopez for his help with the heat treatment of the samples and Ruben Manzanares for his help with assembly. John Bingert and G. T. “Rusty” Gray are also gratefully thanked for the fruitful discussions during the preparation of this manuscript.

Abstract

Plate impact experiments have been carried out to examine the influence of grain boundarycharacteristics on the dynamic tensile response of Cu samples with grain sizes of 30, 60, 100, and 200 μm. The peak compressive stress is ∼1.50 GPa for all experiments, low enough to cause an early stage of incipient spall damage that is correlated to the surrounding microstructure in metallographic analysis. The experimental configuration used in this work permits real-time measurements of the sample free surfacevelocity histories, soft-recovery, and postimpact examination of the damaged microstructure. The resulting tensile damage in the recovered samples is examined using optical and electron microscopy along with micro x-raytomography. The free surfacevelocity measurements are used to calculate spall strength values and show no significant effect of the grain size. However, differences are observed in the free surfacevelocity behavior after the pull-back minima, when reacceleration occurs. The magnitude of the spall peak and its acceleration rate are dependent upon the grain size. The quantitative, postimpact, metallographic analyses of recovered samples show that for the materials with grain sizes larger than 30 μm, the void volume fraction and the average void size increase with increasing grain size. In the 30 and 200 μm samples, void coalescence is observed to dominate the void growth behavior, whereas in 60 and 100 μm samples, void growth is dominated by the growth of isolated voids. Electron backscatter diffraction (EBSD) observations show that voids preferentially nucleate and grow at grain boundaries with high angle misorientation. However, special boundaries corresponding to Σl (low angle, < 5 °) and Σ3 (∼60 ° <111> misorientation) types are more resistant to void formation. Finally, micro x-raytomography results show three dimensional (3D) views of the damage fields consistent with the two dimensional (2D) surface observations. Based on these findings, mechanisms for the void growth and coalescence are proposed.

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Effects of grain size and boundary structure on the dynamic tensile response of copper